Vertical exhaust systems in the form of a stanchion are used on many heavy duty truck configurations. A vertical exhaust system has a natural frequency at which it tends to vibrate. This natural frequency can be attributed to the physical shape and dimension of the system. The natural frequency of a vertical exhaust/stanchion assembly is primarily dictated by its length which is functional in nature and cannot be altered substantially.
During on road operation of a truck, the vertical exhaust systems receive vibration pulses; for example, from engine combustion and road irregularities which substantially increase the rate at which the exhaust system is vibrating. The vibration of vertical exhaust systems are under certain conditions exacerbated by the amplitude of the excitation pulses. For example, when a short wheel base highway tractor is operating with no semi-trailer (a condition known as bob-tail), the truck undergoes harsh excitation when, due to an irregularity in the road, the vehicle is unable to xe2x80x9crecoverxe2x80x9d from a vibration caused by the road irregularity received from the front axle before experiencing a vibration also caused by the road irregularity inputted from the rear axle. Also significant in this situation is the disproportionate spring rate, damping value and hysteresis of the rear suspension which is designed to carry a much greater load than that which is present when in the bob-tail configuration.
Furthermore, the incoming excitation pulses received by a vertical exhaust system are at or near the natural frequency of the vertical exhaust system which causes the vertical exhaust system to approach and sometimes reach resonance. Reaching resonance can result in the destruction of the vertical exhaust systems. As a result, vertical exhaust systems have traditionally had a relatively short life cycle.
The past efforts to increase the longevity of vertical exhaust systems have centered around increasing the stiffness of the system sufficiently to reduce vibrations to within acceptable amplitudes. These efforts have had limited success and do little to solve the resonance problem. As a result, current vertical exhaust systems have a relatively short life cycle and must be replaced periodically during the life of the vehicle.
It has been determined that destructive resonance can develop in vertical exhaust systems due to the fact that the natural frequency, Wn, of the exhaust system corresponds to an excitation frequency, Wf, to which the system is exposed. These excitation frequencies have been attributed to inputs from the road and engine. The natural frequency for vertical exhaust systems for use on heavy duty trucks is about 11 hertz. The excitation pulses that are received from the road fall within a range of 2-11 hertz and engine combustion inputs are in the area of 10 hertz.
Oscillating objects have a natural frequency, which is the frequency that the oscillating object tends to settle into if it is not disturbed. Frequency is the term used to denote the number of times that any regularly recurring phenomenon occurs in one second. The number of cycles per second that an object oscillates is called a hertz (Hz). For example, the natural frequency of a pendulum that is one meter long is 0.5 Hz, which means the pendulum swings back and forth once every 2 seconds. For physical objects, there is usually a relationship between the frequency at which they vibrate (the natural frequency) and the physical dimensions of the vibrating object. Thus, the time required by a pendulum to make one complete swing is partly determined by the length of the pendulum. The shorter the pendulum, the higher will be its natural frequency.
If this pendulum were to be struck lightly once every 2 seconds, the amplitude of the swing would gradually increase and eventually the amplitude of oscillation would become very large. This phenomenon in which a relatively small, repeatedly applied force causes the amplitude of an oscillating system to become very large is called resonance. When an object reaches resonance, serious problems can develop. However, if the proper damping mechanism is utilized, reaching resonance can be avoided. The force that causes vibrations or oscillation to stop is called damping. The most common damping force acting on mechanical mechanisms is friction. However, other damping forces are also available.
This invention is based on the concept that, by applying a spring-mass absorber to a vertical exhaust system, an anti-resonance will be created that will stabilize the vertical exhaust system. The natural frequency of the spring-mass must correspond to the offending resonance frequency of the vertical exhaust system which we are trying to stabilize.
The mass of the spring-mass system has a generally toroidal shape such that it surrounds the vertical exhaust stanchion and is connected thereto by a spring or springs. The term xe2x80x9ctoroidal,xe2x80x9d as used herein, means generally shaped similar to a torus or toroid and is not limited to a closed curve having a specific shape.
Consistent with this concept, the greater the mass, the wider is the range of excitations that it will dampen. However, a vertical exhaust system is basically a cantilevered beam and there is a limit to how much additional mass can be applied to its free end. When the vehicle is stopped, the momentum of the additional mass will tend to bend the vertical exhaust system forward and, when the vehicle accelerates, the additional mass will tend to bend the vertical exhaust system backward. Also, as the stiffness of the spring for the spring-mass absorber increases, the amplitude of the vibrations in the region of excitation also increases, which is undesirable. Thus, the application of the concept to a particular vertical exhaust requires consideration of the specific vertical exhaust system and then compromising and balancing the quantity of the mass and the stiffness of the spring.
This concept has been used, for example, to dampen vibrations in marine structures that are caused by waves striking the structure. Reference may be made to U.S. Pat. No. 5.098,226 for an example where a toroidal absorber tank is installed on the marine structure and tuned to the desired damping frequency by varying the depth of liquid in the tank.
Another example of the use of this concept is in flow regulators where resonance is produced in conduits of certain lengths. To resolve this problem inertial mass is resiliently mounted on the conduit and serves to dampen vibration caused by pressure pulses in the fluid supply conduit. Reference may be made to U.S. Pat. No. 5,853,018 for an example of such a device.